So I'm a week late, but I just watched the first ep of the new BSG season and:
VERY LATE SPOILER AFTER JUMP
Ellen Tigh is the final Cylon.
Ron Moore says he's been planning this for a while, and I believe him. For example, remember that episode in season 2 where Ellen was trying to get a gmail account and had trouble solving the CAPTCHA? It all makes sense now.